Fragile X mental retardation is caused by the absence of expression of the Fragile X Mental Retardation Protein, FMRP. FMRP is an RNA binding protein found in a large, RNA-protein complex called a messenger ribonucleoprotein (mRNP) particle. FMRP is located primarily in the cytoplasm but shuttles into and out of the nucleus. All of these functions - nuclear translocation, RNA binding, and protein-protein interactions - have been shown in other systems to be modulated by phosphorylation. In preparations from rat brain, there is evidence that stimulation of the group 1 metabotropic glutamate receptors (mGluRs) increases expression of rodent FMRP (Fmrp). We propose that mGluR stimulation, which activates kinases, results in the phosphorylation the Fmrp containing-mRNP to modulate its function. We will characterize the role of phosphorylation on Fmrp function through the use of transfected cell lines, purified recombinant Fmrp and neuronal cultures in the following specific aims: 1. Examine the role of tyrosine phosphorylation on the intracellular localization, RNA binding, and protein-protein interactions of Fmrp. 2. Identify serine/threonine kinases that phosphorylate Fmrp and characterize the role of this phosphorylation on the intracellular localization, RNA binding and protein-protein interactions of Fmrp. 3. Characterize the effect of m-GluR-stimulation on the mRNP containing the FmrI mRNA. Understanding how phosphorylation affects Fmrp function, particularly in response to receptor stimulation in the brain, will give insight into the normal function of Fmrp, as well as how its absence leads to mental retardation.